1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus. More specifically, the present invention relates to an optical scanning device that scans a scanning surface with a light beam and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
Optical scanning devices have been widely known in connection with image forming apparatuses, such as optical printers, digital copiers, and optical plotters. With spread of the image forming apparatuses, a demand is increasing for high stability, high definition, and high speed as well as reduction in the number of components and downsizing of the apparatuses.
In recent years, multicolor image forming apparatuses that superimpose a plurality of color images have been developed. In particular, a tandem system, in which a plurality of scanning optical systems forms optical spots on respective photosensitive elements for different colors, is increasingly used to configure the multicolor image forming apparatuses.
To perform writing of multicolor images, a method has been employed in which optical scanning devices are provided for respective colors. In this method, however, there is a problem in that the number of components increases because of provision of optical components for each color; and downsizing becomes difficult because of mounting of a plurality of optical scanning devices inside an image forming apparatus.
Therefore, a system using a single optical scanning device that houses a plurality of scanning optical systems has been proposed. With this system, in general, a plurality of light beams for respective colors enters a single optical deflector, and each light beam is focused onto a corresponding photosensitive drum by each scanning optical system (see FIG. 32).
However, with this system, optical elements for respective colors are closely arranged around the optical deflector, so that downsizing of the optical scanning device becomes limited.
To overcome the limitations on the downsizing of optical scanning devices that handle multiple colors, various methods have been proposed.
For example, there is a known method in which polarization directions of light beams are made different. Two scanning optical systems that are arranged so as to overlap in the vertical direction are integrated. The light beams are split by polarized-beam splitting elements (polarization separation elements) arranged inside the scanning optical systems. Each light beam is guided to a corresponding photosensitive element. Hereinafter, this method is referred to as a “polarization separation system”.
This method is advantageous to reduce the size of an optical scanning device in a direction along the rotation axis of an optical deflector (to make the optical scanning device thin), and to reduce the number of components by sharing optical elements on upper and lower stages in the scanning optical systems.
Other known methods include a method of integrating scanning optical systems by using a dynamic active element, such as a spatial modulation element; a method of splitting a light beam by using a dichroic mirror and light sources with different wavelengths; and a method of causing light beams for all colors to be obliquely incident on a single optical deflector (see Japanese Patent Application Laid-open No. 2004-184591, Japanese Patent Application Laid-open No. 2006-267398, Japanese Patent Application Laid-open No. 2010-26055, and Japanese Patent Application Laid-open No. 2008-15140).
However, when an active element is used, a drive circuit for the active element is needed. When a dichroic mirror is used, different types of light sources are mounted on a single optical scanning device. Therefore, even though the number of optical elements of a scanning optical system is reduced, it is necessary to increase more expensive elements to compensate for the reduction of the optical elements. When a light beam is obliquely incident, optical performance is reduced due to the oblique incidence on an optical deflector and a scanning lens.
To overcome the limitations on the downsizing of optical scanning devices that handle multiple colors, the polarization separation system described above is desirable.
However, with the conventional polarization separation system, it is necessary to ensure a sufficient optical path length in accordance with a write width in order to suppress reduction in optical performance. However, if the optical path length is too long, the layout of the optical path in the optical scanning device becomes limited. Furthermore, when apparatuses are further downsized, optical elements may interfere with each other.